Conventionally, alkyl vinyl ether has been produced by the following methods of two kinds: (1) the so-called Reppe method wherein acetylene in the liquid phase is caused to have addition reaction with alcohol under pressure with use of an alkaline catalyst; and (2) the so-called acetal method wherein acetal which is obtained by dehydration and compression of acetaldehyde and alcohol is de-alcoholated by thermal decomposition in the gas phase.
However, the Reppe method, though exhibiting a high reaction selectivity, has drawbacks such as complexity in reaction control and production control (operation control) including safeguard, from the following reasons: (1) it is an exothermic reaction whereby heat is greatly emitted; (2) acetylene which may possibly decompose and explode has to be dealt with under pressure; and (3) reaction performances such as the reaction selectivity or the reaction yield extraordinarily depend on concentrations of a raw material and a catalyst. On the other hand, the acetal method has drawbacks such as high producing costs, from the following reasons: (1) acetal as a raw material is difficult to obtain and costs high; and (2) an alcohol, which is by-produced to an equal molar number to that of an alkyl vinyl ether, has to be separated from the alkyl vinyl ether and purified, so that the alcohol is recovered to be used as a raw material for synthesis of acetal.
As a method wherein the above drawbacks are solved, the present applicant has before proposed a method for producing alkyl vinyl ether by intramolecular dehydration of glycol ether in the gas phase in the presence of a catalyst (the Japanese Publication for Laid-Open Patent Application No. 8-143497/1996 (Tokukaihei 8-143497)). Glycol ether, which is produced by reaction between alcohol and ethylene oxide, is easy to obtain and does not cost high. As to the producing process, no raw material other than glycol ether is necessary, and moreover, a reaction selectivity of not less than 90 mole percent is achieved by appropriately selecting a type of a catalyst and reactive conditions.
A gaseous reaction mixture (hereinafter referred to as reaction gas) obtained through the above process contains as by-products water, acetaldehyde and alcohol which are products of decomposition of glycol ether, and non-reacted glycol ether, in addition to alkyl vinyl ether as an object.
Alkyl vinyl ether generally has the following properties: (1) alkyl vinyl ether is an oleaginous compound which does not dissolve in water; (2) alkyl vinyl ether forms an azeotrope with water; (3) alkyl vinyl ether forms an azeotrope with an alcohol having an alkyl group in the same structure of that of the alkyl group of the alkyl vinyl ether (for example, alcohol produced by side reaction); and (4) since a vinyl group of alkyl vinyl ether has high reactivity, addition reaction wherein a compound having an acidic hydrogen atom or a basic hydrogen atom is added to the vinyl group tends to occur, and hence its properties easily change. Furthermore, glycol ether as a raw material and alcohol which is produced by side reaction generally have a property of forming an azeotrope with water.
Therefore, a reaction gas obtained through the aforementioned process is a mixture of compounds which have properties (physical properties) complicatedly relating to each other. For industrially and efficiently producing alkyl vinyl ether, it is necessary to efficiently liquidize and collect alkyl vinyl ether from a reaction gas, and further, to purify and recover alkyl vinyl ether and glycol ether which are raw materials.
However, the aforementioned publication does not particularly teach a method for efficiently collecting, purifying, and recovering alkyl vinyl ether from the reaction gas.
As to the reaction gas, an azeotrope of alkyl vinyl ether and water, though not always but depending on the type of alkyl vinyl ether, has an azeotropic point which is the lowest after the boiling point of acetaldehyde. In other words, it is difficult to liquidize alkyl vinyl ether, and hence, alkyl vinyl ether cannot be efficiently collected simply by liquidizing the reaction gas. Therefore, it is necessary to collect alkyl vinyl ether by diluting the reaction gas with a compound having a relatively high boiling point as diluent.
Furthermore, in the case where the reaction gas is collected with use of a collecting agent and the agent is a liquid compound widely used, such as an aliphatic-group hydrocarbon, an aromatic-group hydrocarbon, an ester of carbonic acid, or water, a solution after collection (hereinafter referred to as collected liquid) exhibits phase separation, separating into an oleaginous layer and an aqueous layer. Then, alkyl vinyl ether selectively dissolves in the oleaginous layer, while non-reacted glycol ether divides into two at a predetermined ratio thereby dissolving in the two layers respectively. Therefore, in the case where the reaction gas is collected with the use of the liquid compound widely used, the recovering operation of glycol ether has to be conducted with respect to both the oleaginous and aqueous layers to re-use glycol ether, and hence steps for recovery increase, thereby impairing productivity of the alkyl vinyl ether. Note that a chemical compound having an acidic hydrogen atom or a basic hydrogen atom cannot be used as the collecting agent since addition reaction occurs between alkyl vinyl ether and such compounds.
Furthermore, if distillation separation is simply conducted with respect to the collected liquid in order to purify and recover alkyl vinyl ether and glycol ether from the collected liquid, the compounds are distilled in an order from that having the lowest boiling point to that having the highest boiling point. More specifically, if the collected liquid is distilled, the following compounds are obtained in the following order: (1) acetaldehyde; (2) an azeotrope of alkyl vinyl ether and water; (3) an azeotrope of alcohol and water; (4) an azeotrope of glycol ether and water; and (5) glycol ether. Thus, if distillation separation is simply conducted, glycol ether is last obtained, and hence it takes long to conduct the distillation, thereby impairing productivity of alkyl vinyl ether. Besides, glycol ether, which is distilled together with water, cannot be recovered and re-used, since it is difficult to separate glycol ether from water. Therefore, in this case, a recovery of glycol ether decreases, and as a result, a yield and productivity of alkyl vinyl ether are further impaired.
Therefore, to establish an alkyl vinyl ether producing method suitable for industrial application, a method for efficiently collecting, purifying, and recovering alkyl vinyl ether as an object and glycol ether as non-reacted raw material from the reaction gas is earnestly demanded.